Consider the police or feds paying auto-repair shops to examine cars for contraband while under repair, and have the cops pull you over as soon as you pick up your car.
If you rent, your landlord has the key to your apartment and is often allowed by the rental contract to inspect the apartment. Can he plant bugs for the feds while there?
Anything that is improper for "law enforcement" to do directly, is improper when done by a paid third-party.